302 DR. H. LYSTER JAMESON ON 
margin, and arises in the normal manner in a marginal groove 
(Ceylon Report, Part If. Anatomy of Pearl-Oyster, plate vin. 
fig. 2 
Text-fig. 35. 












ANINTatt ; 
Marga ritifera vulgaris, Persian Gulf. Section through the lip of the shell, after 
decalcification, showing the successive lappet-like processes of the prismatic 
substance (2.,7./, 1.7, 0./’’). pr. prismatic substance ; Sév., stratification of same. 
iA AS, A’, A’”’, points marking the successive retreats of the secreting margin, 
which takes place when new lappets are to be formed. At these points the 
‘ periostracum ” of the new lappet is continuous with, and indistinguishable 
from, the inner conchyolin-layer of the prismatic substance of the last-formed 
lappet. Preparation X (xX 35), see also Pl. XXXIV. fig. 6 
(ii.) Zhe Prismatic Substance. 
Reduced to its simplest terms the prismatic layer of the 
Mother-of-Pearl shell consists of prisms of calcium carbonate 
perpendicular to the surface of the shell, bounded externally and 
internally by membranes of conchyolin, which are connected by 
vertical membranes forming the septa between the prisms 
(text-figs. 35 and 36; see also Pl. XXXIV. figs. 6, 6a; Pl. XL. 
fig. 29a). Where interruptions in the continuity of the growth 
of this layer have occurred, the layer of prisms may be divided 
by one or more horizontal walls of conchyolin (text-figs. 35, str., 
and 38, str.), which break up the prisms into segments, or which 
divide the layer into two or more series of prisms. The individual 
ends of the prisms in one layer do not of necessity coincide with 
those of the prisms in the next layer, though they frequently do 
coincide. The septa between the prisms may also show annular 
thickenings, corresponding to zones of constriction around the 
prisms (Pl. RK LY. fig. 6 a,ann.). Romer has shown (82, p. 35) 
that the prisms of the pearl- -shell (like those of Pinna, described 
by Biedermann (1), p. 9) behave between crossed nicols in the 
same manner as single crystals. The prisms differ enormously in 
size, according to the age of the oyster and the conditions under 
which they are secreted. 
There is a sharp line of demarcation between the prismatic 
and the nacreous layers in Margaritifera, the innermost layer of 
the conchyolin of the former being connected to the outermost 
layer of that of the latter by a series of fine connectives of 
conchyolin (Pl. XXXTV. fig. 6 a, con.), forming a curious alveolar 
layer. 
[44] 
